fix links in docs by re-adding .html suffix

[RafalSkolasinski]

What this PR does / why we need it:

Fix multiple broken links in documentation by re-adding .html suffix

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[seldondev]

Wed Oct 14 16:37:47 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2549/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2549 --build=1

[seldondev]

Wed Oct 14 16:37:48 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2549/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2549 --build=2

[axsaucedo]

/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

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